Boiler

ABSTRACT

A water tube boiler has rows of inner and outer riser tubes extending along opposite sides of a furnace area. A burner opens into one end of the furnace area. The areas between adjacent riser tubes in each row are closed except in the inner row where a number of such areas are left open to the furnace area at the burner end of the furnace area. A refractory tunnel extends from the burner into the furnace area beyond the open areas in the inner tube rows. In an alternative embodiment two additional rows of riser tubes extend into the furnace area beyond the open areas in the inner tube rows with the areas between adjacent tubes of the additional rows being closed.

' United States Patent 15] 3,693,598 Cleaver [451 Sept. 26, 1972 [54i BOILER Primary Examinerl(enneth W. Sprague [72] Inventor: John C. Cleaver, Milwaukee, Wis. wiviott and Ralph Hohenfeldt [73] Assignee: Aqua-Chem, Inc Milwaukee, Wis. 571 ABSTRACT Filed: Julie 29, 1971 A water tube boiler has rows of inner and outer riser App]. No.: 157,981

US. Cl ..122/274, 122/347 Cleaver et al 122/235 tubes extending along opposite sides of a furnace area. A burner opens into one end of the furnace area. The areas between adjacent riser tubes in each row are closed except in the inner row where a number of such areas are left open to the furnace area at the burner end of the furnace area. A refractory tunnel extends from the burner into the furnace area beyond the open areas in the inner tube rows. In an alternative embodiment two additional rows ofriser tubes extend into the furnace area beyond the open areas in the inner tube rows with the areas between adjacent tubes of the additional rows being closed.

23 Claims, 6 Drawing Figures PATENTEDsms I972 SHEET 2 [1F 4 OOQOA MU Gm 0&22

PATENTEDSEPZS 1912 SHEET 0F 4 w w 0 4 a a 0 a 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0-0 00 0 0 0-0 0 0-0 0 0-0 0 O O O O O O O O 0 6 O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O OOOOOOO OOOOOOOOOOOOOOOOOOA 0 0 0 0; & O O O O OO I. M

O O O 0 0 O O O O O O O O OO O O OO O O O OO O OOOOOOOOOOOOOOOOOO O O O O O O 0 0 O O O O O OO O O OO O O O 0 0 O r I I I i J w J 1 BOILER BACKGROUND OF INVENTION SUMMARY OF INVENTION This invention has among its general objects the provision of a boiler construction which achieves one or more of the following: complete combustion in the furnace area of the boiler; effective utilization of available heat; a versatile boiler design; and a compact and yet more efficiently operating boiler.

For the achievement of these and other objects this invention proposes a boiler construction wherein at least two laterally spaced rows of riser tubes extend along opposite sides of a furnace area of the boiler. A burner assembly opens into the furnace area at one end thereof. and the inner row of riser tubes (i.e., inner with respect to the furnace area) have a number of adjacent tubes in the area of the burner assembly which are spaced apart and have an opening therebetween. The remainder of the inner row is closed to the furnace in the area between adjacent riser tubes. The areas between adjacent riser tubes in the outer row are closed so that a passage is defined between the rows which is open to the furnace area only through the open area, or areas, between the inner tubes at the burner assembly end of the furnace area. An exhaust duct communicates with the defined passage at the opposite end of thetube rows (i.e., opposite from the burner assembly end) to permit escape of the combustion gases, etc. The flame issuing from the burner assembly penetrates the furnace area and, since the only avenue of escape from that area is through the inner tube row open areas at the burner assembly end of the furnace area, the flame, together with any unburned products of combustion, must turn back on itself to exit the furnace area. This tum-back pattern contributes to more complete combustion, more effective heat utilization, the compactness of the boiler, and an overall more efficiently operating boiler.

To improve even further on these advantageous results, certain additional specific features may also be incorporated in the boiler construction. For example, a refractory tunnel and/or additional riser tubes may be associated with the burner assembly and project into the furnace area beyond the open areas of the inner tube row. The tunnel, when used, assists in imparting a velocity to the flame sufficient to penetrate the length of the furnace area. By extending beyond the open areas, the refractory tunnel and/or the additional riser tubes reduce the likelihood of direct exit of the flame without penetrating the furnace area.

Also, the tubes of adjacent rows can be staggered which, in addition to allowing a more compact tube arrangement, defines a tortuous path to insure increased and more effective contact between the hot gases and the tubes as the gases move through the defined passage.

In addition, the boiler can be provided with a single lower tank and a single upper tank connected by the rows of riser tubes, and downcomer tubes, with the tanks and tubes being symmetrical about the vertical centerline of the boiler. This basic design thus lends itself to the construction of dilferent boiler capacities with use of riser tube rows on either one or both sides of the boiler as desired. If desired, riser tubes can also be provided between the basic inner and outer tube I'OWS.

Other objects and advantages will be pointed out in or be apparent from the specification and claims, as will obvious modifications of the embodiment shown in the drawings, in which:

FIG. I is a perspective view of a part of the boiler illustrat'ing the flow pattern;

FIG. 2 is an axial section of the boiler;

FIGS. 3 and 4 are views of the opposite ends of the boiler;

FIG. 5 is a top view, in section, of the boiler; and

FIG. 6 is a top view, in section, of an alternative embodiment of the boiler.

DESCRIPTION OF PREFERRED EMBODIMENT This invention will perhaps find its widest utility in connection with water tube boilers, and for that reason, has been illustrated and will be described as embodied in a water tube boiler.

With particular reference to the drawings a water tube boiler 10 has been illustrated. For ease of illustration and description, the outer casing of the boiler has been removed in FIG. I. The basic boiler structure includes a lower tank 12 and an upper tank 14 connected, in a manner to be described more completely hereinafter, by riser tubes and downcomer tubes. Angle iron members 16 and 18 are welded to lower tank 12 and cooperate in forming the support base for the overall boiler. Suitable fill and outlet openings and attachments can be provided in upper tank 14, this can be done in a conventional manner and hence will neither be illustrated nor described in detail.

The boiler is constructed so as to be symmetrical about a longitudinal, vertical plane through the boiler including the axis or vertical centerline of both the upper and lower tanks. Both sides of the boiler are identical and thus the riser and downcomer tube arrangements will be described on only one side of the boiler but it will be appreciated that the description is equally applicable to the other side. To assist in following the description on both sides of the boiler, similar numbers will be attached to elements on both sides but with the subscript a used for that side to which direct reference is not being made.

With reference to the front side of the boiler (front as viewed in FIGS. 1 and 2) a plurality of inner riser tubes 20 extend along one side of the boiler. Similarly, a plurality of outer riser tubes 22 extend along the same side of the boiler. Tubes 20 and 22 are arranged in inner and outer rows relative to the interior of the boiler. Lower tank 12 includes a plurality of aligned openings 24 into which riser tubes 20 fit and a second row of aligned openings 26 into which riser tubes 22 fit. Similarly, upper tank 14 is provided with aligned openings 28 and 30 which receive riser tubes 20 and 22, respectively. As will appear more clearly as this description proceeds, the riser tubes extend along opposite sides of furnace area 31 of the boiler.

' A'burner assembly 32 is arranged at the right end (as viewed in FIGS. 1 and 2) of the furnace area and includes a burner 34 disposed in a plenum chamber 36. The plenum chamber can alsoinclude an air supply fan (not shown) and the burner is supplied with fuel from a fuelsource and pump (also not shown). The mixture of fuel and air is ignited at the burner and directed into the furnace area. Air is forced through plenum chamber 36 and directed into the furnace area through a diffuser 38 located at the burner. The diffuser attributes a swirling action to the flame but being of con? ventional construction will not be described in detail.

Except for theopening 33 of the burner assembly into the furnace area, the right end of the furnace area is otherwise closed. More specifically, auxiliary riser tubes 40, 42, and 44, communicate with each other and the end riser tube 20, with the areas between the main riser tube and the auxiliary riser tubes being closed by webs 46, 48, and 50 which are welded to their adjacent riser tubes. A central web section 52 closes the area between the adjacent auxiliary riser tube arrangements. This arrangement closed the right end of the furnace area. It also provides riser tubes across the right end so that available heat transfer in that area is not lost.

Similarly the left end of the furnace area is closed. Structurally, the closure of that end of the furnace area is accomplished by a removable assembly 54 in direct alignment with burner assembly 32. The inner face of assembly 54 is made up of a refractory block 56 upon which the flame penetrating the furnace area from the burner assembly impinges directly. The entire assembly 54 is releasably attached to brackets 58 and60 by nut and bolt combinations 62. Removable assembly 54 permits access into the furnace area for maintenance, inspection, etc. The remainder of the left end of the furnace area is closed again by an auxiliary riser tube assembly. More particularly, and with reference to FIG. 3, auxiliary risertubes 64, 66, and 68 communicate with each other and main riser tube. 20. Webs 70, 72, and 74 close the area between the auxiliary riser tubes and the main riser tube and central web 76 closes the area between the adjacent sets of auxiliary riser tubes.

Thus both ends of the furnace'area are closed with the exception ofthe opening of the burner assembly into the furnace area.

As is perhaps most clearly illustrated in FIG. 5, inner riser tubes 20 are relatively spaced apart in the inner row. Metal webs 78 are welded between the inner tubes except for a plurality of tubes at the right or burner assembly end of -the boiler. This arrangement provides openings 80 in the area of the burner assembly which communicate with the furnace area.

Tubes 22 of the outer row are also relatively spaced apart and a metal web 82 is welded between each pair of adjacent outer tubes throughout the entire length of the outer row. End webs 84 and 86 extend between the inner and outer tubes at both ends of the tube rows. With this arrangement, the inner and outer tube rows define a passage 88, and 88a, between the tube rows which is open to the furnace area only through openings 80, or 80a,,on the opposite sides of the furnace area.

With the arrangement described to this point, it will be noted that the flame issuing from burner assembly 32 together with other products of combustion,

penetrates the furnace area 30 impinging on the left end of the furnace area but must turn back on itself to exit the furnace area through openings and 80a. The

series of arrows in FIGS. 1 and 2 illustrate this flow pattern.

An exhaust stack 90 communicates with the passageway on both sides of the boiler and the hot gases flowing through the passageways 88 and 88a exit those passageways through stack 90. As illustrated in FIG. 1, the stack 90 includes outer housing 92 and communication between the stack and the passageways is achieved by omitting the upper portion of web 82 between portions of outer riser tubes 22 at the left end of the boiler. Thus the flame must not only turn back on itself in the furnace area but must traverse the entire length of passages 88 and 88a before exiting through exhaust stack 90.

With this defined flow path for the products of combustion, virtually complete combustion is assured within the furnace area. In other words, as a result of the turn-back pattern the likelihood of unburned products of combustion is reduced because the circulating gases are caused to be re-exposed to the high temperature flame emanating from the burner assembly and because it is less likely that burning particles will contact the surfaces defining the furnace area and be quenched. Also, this means the flue gases will contain less undesirable emissions.

In addition to enhancing the chances of complete combustion, the turn-back pattern of the hot gases has proved to afford additional advantages. One additional advantage is that by maintaining the hot gases in the furnace area longer the available radiant heat in the furnace area is better utilized. In this connection, it has been noted that the gases as they enter the passageway between the tubes are at a relatively lower temperature than would be the case if they were to exit the furnace area at the opposite end. For example, in some instances where the exit gases might otherwise be at a temperature of approximately 2,000 F it has been found that the temperature of the exiting gases passing through openings 80 and 80a are approximately l,500-1,600 F. Further significance in this reduced temperature is that the hot gases approaching the relatively cooler riser tubes have a lower temperature differential relative to the tubes thereby reducing the possibility of quenching as hot gases contact the cooler metal surfaces. Thus, there is less likelihood that any products entrained in the hot gases will still be burning as they enter the tubes and a quenching effect, with possible attendant deposits on the tubes, will not take place.

Furthermore, with the defined pattern the flame is well enough distributed to avoid hot spots in the furnace area. More specifically the flame pattern reduces thetendency of the flame to impinge directly on the tube structure at the left end of the boiler and hence undue heating of that tube structure will not occur. Also, the fact that the boiler has a membrane construction i.e., webs 78 and $2 welded between adjacent tubes) contributes to the elimination of hot spots in that the webs will be cooled by the tubes thereby eliminating them as a source of hot spots.

All of these features contribute, in some measure, to allowing the boiler to be fired at a higher temperature while maintaining effective and efficient operation as well as an overall compact size.

With particular reference to FIG. 2, it can be seen that burner assembly 32 includes a tunnel 94 which extends into the furnace area. The tunnel is made of suitable refractory material and together with the remainder of the'burner defines a combustion chamber through which the flame and hot gases emerge into the furnace area. The interior of the tunnel is provided with a generally conically shaped opening 96 tapering inwardly'from its outer end 98 toward the burner. The conically shaped opening together with the diffuser and the air supply mechanism of the burner assembly contribute to assure that the flame and hot gases enter the furnace area with a velocity sufficient to impinge on the left wall and turn back for exit through the passageways 88 and 880. It will further be noted (see FIG. 5) that the refractory tunnel 94 extends into the furnace area beyond the openings 80 and 80a and is provided with a marginal flange 100. This overlapping extension of the refractory tunnel with respect to the openings prevents direct passage of the flame and/or hot gases from the burner assembly through the openings 80 and 80a and assures that the hot gases and flames must penetrate the furnace area. Flange 96 also contributes in this respect.

it will also be noted that openings 24 and 26 (and 24a and 26a) are staggered with respect to each other. Thus, inner tubes 20 and outer tubes 22, (and 20a and 22a) are also staggered with respect to each other. With this staggered arrangement, a generally tortuous path is attributed to passageways 88 and 88a. This tortuous path has the effect of increasing the contact between the hot gases and the riser tubes. Also, the tortuous path prevents direct passage of the hot gases through the passageways and insures a longer period of contact between the gases and the riser tubes. Both of these features increase the heat transfer between the hot gases and the riser tubes.

To complete the basic structure of the boiler, a floor 102 of refractory material is provided over lower tank 12. Also, downcomer tubes 104 and 104a extend between openings 106 and 108 in the upper and lower tanks respectively- The downcomer tubes are positioned outside of the wall defined by removable insert 54 and the auxiliary riser tube-web assemblies, so as to isolate the downcomer tubes from the direct heat in the furnace area. A sight glass assembly 110 can be provided at the upper tank and communicates with the tank through couplings 112 and 114 to give a visual reading of the water level in the upper tank.

In operation, water will circulate between lower tank 12 and upper tank 14 by hot water, or steam, rising in the riser tubes and returning to the lower tank through the downcomer tubes by means of convective circulatlOIl In the alternative embodiment illustrated in FIG. 6, boiler .120 includes inner tube rows 122 and 122a and outer tube rows I24 and 1240. Theserows, as in the previously described embodiment, consists of riser tubes the areas between which are closed by metallic webs. The entire outer tube rows are closed whereas openings 1 26 and 126a are provided the inner tube rows adjacent the end of the boiler at which burner assembly 128 is located. The opposite ends 13.0 and 132 of the boiler are closed by a riser tube-web assembly. The connection to the stack of the boiler is made through chambers 34 and 134a.

Additional rows of riser tubes are arranged in inner rows 136 and 136a. These rows extend from burner wall inwardly of the combustion area of the boiler to a point beyond openings 126 and 126a in tube rows 122 and 1220 and are closed to combustion area along their extension. In this arrangement, the products of combustion are again directed into the combustion area but must turn back on themselves to exit through openings 126 and 1260 and the passage defined between tube rows 122-124 and 12211-1240. The capacity of the boiler is increased by using the additional riser tube rows 136 and 136a. Also, if desired, additional riser tubes 138 and 138a can be dispersed through the passages between tube rows 122-124 and 122a-124a.

The riser tubes can be connected, in the same manner as any previously described embodiment to upper and lower tanks so that, with the use of suitably arranged downcomer tubes, circulation occurs by means of convection.

An additional advantage resulting from the boiler construction which has been described is that the overall size can be reduced without sacrificing output. Specifically, it will be noted that the overall length of the boiler can be reduced without sacrificing heat transfer. In addition, other advantages are also achieved, such as more complete combustion, increasing exposure of the hot gases by passing the hot gases through adjacent rows of riser tubes, reducing the likelihood of deposits on the tubes and thereby reducing the necessity for cleaning, and providing a symmetrical modular type boiler construction which permits various capacities within a basic boiler design.

Although but one embodiment of the present invention is illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit of this invention or the scope of the appended claims.

Iclaim:

1. A water tube boiler comprising, in combination,

a furnace area having opposed ends,

an upper tank and a lower tank,

at least two adjacent rows of riser tubes extending along said furnace area and arranged such that there is an inner row of tubes and an outer row of tubes relative to said furnace area,

a burner assembly at one end of and operatively opening into said furnace area, said one end being otherwise closed to said furnace area and the opposite end of said furnace area being closed,

and means defining an opening in said inner tube row in the area of said one end of said furnace area and means closing the area between adjacent tubes in the remainder of said inner tube row to flow through the area between adjacent tubes,

means closing the area between adjacent tubes in said outer tube row to flow through the area between adjacent tubes so that said inner and outer tube rows define a passageway therebetween open to said furnace area only in the area of said one furnace area end,

and means defining an exhaust communicating with said passageway,

7 whereby products of combustion penetrate said furnace area from said burner assembly and turn back on themselves to exit the furnace area and enter said passageway through the openings between saidtubes in said inner tube row. 2. The water tube boiler of claim 1 including a second arrangement of at least two adjacent rows of riser tubes extending along said furnace area opposite to said first mentioned rows of riser tubes and also arranged in an inner row of tubes and an outer tow of tubes relative to said furnace area, and in said second arrangement means defining an opening in said inner tube row means in the area of said one end of said furnace area and means closing the area between adjacent tubes in the remainder of said inner tube row to flow through the area between adjacent tubes, means closing the area between adjacent tubes in said outer tube row to flow through the area between adjacent tubes so that said inner and outer tube rows define a passageway therebetween open to said furnace area only in the area of said one furnace area end, I and means defining an exhaust communicating with said passageway. 3. The water tube boiler of claim 2 wherein said burner assembly has an extension into said furnace beyond the tubes in said inner tube rows having the opening into said furnace area and being open to said furnace area at the end thereof and closed to said furnace area along the extension thereof.

4. The water tube boiler of claim 3 wherein said extension of said burner assembly comprises a refractory tunnel.

5. The water tube boiler of claim 4 wherein the interior of said refractory tunnel is generally conical in axial cross section.

6. The water tube boiler of claim 2 including two rows of inner riser tubes extending-along but spaced from said inner row of riser tubes and terminating in spaced relation from said opposite end of said furnace area and means closing the areas between the tubes in the rows, of inner riser tubes.

7. The water tube boiler of claim 6 including riser tubes in the areas between said inner and outer tube rows.

8. The water tube boiler of claim 1 wherein said burner assembly has an extension into said furnace beyond the tubes in said inner tube row having the opening into said furnace area and being open to said furnace area at the ends thereof and closed to said furnace area along the extension thereof.

9. The water tube boiler-of claim 7 wherein said extension of said bumerassembly comprises a refractory tunnel the interior of which is generally conical in axial cross section.

10. The water tube boiler of claim 1 including riser tubes in the areas between said riser and outer tube rows.

11. The water tube boiler of claim 2 wherein the 6 tubes of said inner and outer rows are staggered as between rows to thereby attribute a generally tortuous configuration to said passageways.

12. The water tube boiler of claim 1 wherein the tubes of said inner and outer rows are staggered as between rows to therebyattribute a generally tortuous configuration to said passageways.

13. The water tube boiler of claim 2 wherein said exhaust opens into said passageways in the area of the opposite end of said furnace area. I

14. The water tube boiler of claim 1 wherein said exhaust opens into said passageway in the area of the opposite end of said furnace area.

15. The water tube boiler of claim 2 wherein said upper and lower tanks and said rows of riser tubes are arranged symmetrically about a central vertical plane through said boiler.

16. The water tube boiler of claim 1 including at least one downcomer tube extending between said upper and lower tanks outside of said one end of said furnace area. i

17. The water tube boiler of claim 2 wherein said means closing the areas between adjacent tubes comprises metal webs extending between and connected to adjacent tubes.

18. The water tube boiler of claim 1 wherein said means closing the areas between adjacent tubes comprises metal webs extending between and connected to adjacent tubes.

19. A water tube boiler comprising, in combination,

a furnace area having opposite ends and sides,

an upper tank and a lower tank,

at least two adjacent rows of riser tubes extending along each side of said furnace area and, at each side and with respect to said furnace area, arranged in an inner row of tubes and an outer row of tubes,

a burner assembly at one end of said furnace area extending into said furnace area and'being closed to the furnace area along the extension thereof,

a plurality of adjacent tubes in said inner tube rows and in the area of said one furnace area and being relatively spaced and having an opening therebetween and means closing the area between adjacent tubes in the remainder of said inner tube rows to flow through the area between adjacent tubes,

means closing the area between adjacent tubes in said outer tube rows to flow through the area between adjacent tubes so that said inner and outer tube rows define passageways therebetween open to said furnace area only in the area of said one furnace area and,

the extension of said burner assembly into said furnace area being beyond said inner row tubes having the openings therebetween,

and means defining an exhaust communicating with said passageways,

whereby products of combustion penetrate said furnace area from said burner assembly and turn back on themselves to exit the furnace area and enter said passageways through the openings between said tubes in said inner tube rows.

20. The water tube boiler of claim 19 wherein said extension of said burner assembly comprises a refractory tunnel the interior of which is generally conical in axial cross section.

furnace area.

23. The water tube boiler of claim 19 wherein said means closing the areas between adjacent tubes comprises metal webs extending between and connected to adjacent tubes. 

1. A water tube boiler comprising, in combination, a furnace area having opposed ends, an upper tank and a lower tank, at least two adjacent rows of riser tubes extending along said furnace area and arranged such that there is an inner row of tubes and an outer row of tubes relative to said furnace area, a burner assembly at one end of and operatively opening into said furnace area, said one end being otherwise closed to said furnace area and the opposite end of said furnace area being closed, and means defining an opening in said inner tube row in the area of said one end of said furnace area and means closing the area between adjacent tubes in the remainder of said inner tube row to flow through the area between adjacent tubes, means closing the area between adjacent tubes in said outer tube row to flow through the area between adjacent tubes so that said inner and outer tube rows define a passageway therebetween open to said furnace area only in the area of said one furnace area end, and means defining an exhaust communicating with said passageway, whereby products of combustion penetrate said furnace area from said burner assembly and turn back on themselves to exit the furnace area and enter said passageway through the openings between said tubes in said inner tube row.
 2. The water tube boiler of claim 1 including a second arrangement of at least two adjacent rows of riser tubes extending along said furnace area opposite to said first mentioned rows of riser tubes and also arranged in an inner row of tubes and an outer tow of tubes relative to said furnace area, and in said second arrangement means defining an opening in said inner tube row means in the area of said one end of said furnace area and means closing the area between adjacent tubes in the remainder of said inner tube row to flow through the area between adjacent tubes, means closing the area between adjacent tubes in said outer tube row to flow through the area between adjacent tubes so that said inner and outer tube rows define a passageway therebetween open to said furnace area only in the area of said one furnace area end, and means defining an exhaust communicating with said passageway.
 3. The water tube boiler of claim 2 wherein said burner assembly has an extension into said furnace beyond the tubes in said inner tube rows having the opening into said furnace area and being open to said furnace area at the end thereof and closed to said furnace area along the extension thereof.
 4. The water tube boiler of claim 3 wherein said extension of said burner assembly comprises a refractory tunnel.
 5. The water tube boiler of claim 4 wherein the interior of said refractory tunnel is generally conical in axial cross section.
 6. The water tube boiler of claim 2 including two rows of inner riser tubes extending along but spaced from said inner row of riser tubes and terminating in spaced relation from said opposite end of said furnace area and means closing the areas between the tubes in the rows of inner riser tubes.
 7. The water tube boiler of claim 6 including riser tubes in the areas between said inner and outer tube rows.
 8. The water tube boiler of claim 1 wherein said burner assembly has an extension into said furnace beyond the tubes in said inner tube row having the opening into said furnace area and being open to said furnace area at the ends thereof and closed to said furnace area along the extension thereof.
 9. The water tube boiler of claim 7 wherein said extension of said burner assembly comprises a refractory tunnel the interior of which is generally conical in axial cross section.
 10. The water tube boiler of claim 1 including riser tubes in the areas between said riser and outer tube rows.
 11. The water tube boiler of claim 2 wherein the tubes of said inner and outer rows are staggered As between rows to thereby attribute a generally tortuous configuration to said passageways.
 12. The water tube boiler of claim 1 wherein the tubes of said inner and outer rows are staggered as between rows to thereby attribute a generally tortuous configuration to said passageways.
 13. The water tube boiler of claim 2 wherein said exhaust opens into said passageways in the area of the opposite end of said furnace area.
 14. The water tube boiler of claim 1 wherein said exhaust opens into said passageway in the area of the opposite end of said furnace area.
 15. The water tube boiler of claim 2 wherein said upper and lower tanks and said rows of riser tubes are arranged symmetrically about a central vertical plane through said boiler.
 16. The water tube boiler of claim 1 including at least one downcomer tube extending between said upper and lower tanks outside of said one end of said furnace area.
 17. The water tube boiler of claim 2 wherein said means closing the areas between adjacent tubes comprises metal webs extending between and connected to adjacent tubes.
 18. The water tube boiler of claim 1 wherein said means closing the areas between adjacent tubes comprises metal webs extending between and connected to adjacent tubes.
 19. A water tube boiler comprising, in combination, a furnace area having opposite ends and sides, an upper tank and a lower tank, at least two adjacent rows of riser tubes extending along each side of said furnace area and, at each side and with respect to said furnace area, arranged in an inner row of tubes and an outer row of tubes, a burner assembly at one end of said furnace area extending into said furnace area and being closed to the furnace area along the extension thereof, a plurality of adjacent tubes in said inner tube rows and in the area of said one furnace area and being relatively spaced and having an opening therebetween and means closing the area between adjacent tubes in the remainder of said inner tube rows to flow through the area between adjacent tubes, means closing the area between adjacent tubes in said outer tube rows to flow through the area between adjacent tubes so that said inner and outer tube rows define passageways therebetween open to said furnace area only in the area of said one furnace area and, the extension of said burner assembly into said furnace area being beyond said inner row tubes having the openings therebetween, and means defining an exhaust communicating with said passageways, whereby products of combustion penetrate said furnace area from said burner assembly and turn back on themselves to exit the furnace area and enter said passageways through the openings between said tubes in said inner tube rows.
 20. The water tube boiler of claim 19 wherein said extension of said burner assembly comprises a refractory tunnel the interior of which is generally conical in axial cross section.
 21. The water tube boiler of claim 20 wherein the tubes of said inner and outer rows are staggered as between rows to thereby attribute a generally tortuous configuration to said passageways.
 22. The water tube boiler of claim 19 including at least one downcomer tube extending between said upper and lower tanks outside of said one end of said furnace area.
 23. The water tube boiler of claim 19 wherein said means closing the areas between adjacent tubes comprises metal webs extending between and connected to adjacent tubes. 